What happens if a person who isn't depressed is on antidepressents?

Not Much. There is no specific risk if a person takes antidepressants but does not need them. The risk of side effects is the same. There will just not be any particular benefit from the medication.
Reason. Perhaps the person is taking antidepressants for anxiety disorders or off label for insomnia or pain control. If the person just ingests antidepressants for no condition it will not make them feel happier but it would subject them to possible side effects.